Presently known abrasive foam grinding structures are known as "honing stones" and typically have open cell structures or closed cell structures. Stones having an open cell structure are generally made from a combination of Polyvinyl alcohol (PVA), starch, and a silicon carbide filler functioning as the abrasive material. An appropriate catalyst, for example sulfuric acid and/or formaldehyde, is added to the PVA, starch, and silicon carbide mixture to convert the mixture to a rubber-like mass with the starch particles randomly entrained therein.
The rubber-like mass is then flushed in a hot water shower to dissolve and flush out the starch from the composite. Upon flushing the starch from the rubber-like mass, a random distribution of holes having diameters in the range of about 50 to 150 microns is formed within the mass, creating a spongy material. The spongy material may then be impregnated with a stiffening agent, e.g. melamine, which penetrates through the random distribution of holes, coating the complex surfaces within the spongy material. Excess stiffening agent which accumulates within the holes may then be removed, for example, through centrifugal extraction or a subsequent rinsing process. The resulting stiffened spongy material is then dried and used to lap or grind the surfaces of workpieces (e.g. computer hard disks).
An alternative stiffening technique useful in open cell structures involves adding various co-polymers to the PVA before curing to enhance the stiffness of the finished spongy material.
Stones having closed-cell structures are typically produced by mixing a "Part A" and a "Part B" urethane resin together. With this process, one of the two parts is used to disperse an abrasive, such as silicon carbide, throughout the resin. While "Part A" and "Part B" are stored separately prior to mixing, upon being mixed together the composite resin mixture is quickly introduced into an injection mold, whereupon a spontaneous exothermic cross-linking reaction occurs. The exothermic reaction between the two resins generates a substantial amount of heat, which expands the resin blowing agents present within one or both of the resin components, forming foam or high density bubbles within the compound.
The blowing agents may be either chemical or physical. Chemical blowing agents generate gas through a chemical reaction, while physical blowing agents generate vapors upon exposure to heat or reduction in pressure. Accordingly, bubbles produced by the gases or vapors from the blowing agents become entrained within the cross-linked matrix, resulting in a porous, closed cell structure suitable for lapping or honing the surface of the workpieces.
When used to hone or lap workpieces, open-cell stones typically flake, liberating particulates at the stone-workpiece interface. To remove the stone particulates and particulates liberated from the workpiece, and to cool the interface which is heated by the grinding or polishing process, the workpieces are typically flushed with water or a water-based solution during processing. Because of the open cell structure, some of the particulates may penetrate into the stone's cell structure during flushing, resulting in "loading" of the debris within the stone. Consequently, the accuracy and precision of the workpiece honing process may be affected, for example, local stiffness deviations across the stone surface formed as a result of "loading".
Problems may also arise from incomplete mixing of "Part A" and "Part B" of the resin. Incomplete mixing may create non-homogeneous or insufficiently homogenous regions in the stone, leading to a non-uniform stone surface. A nonuniform stone surface, in turn, can cause uneven material removal from the workpiece, as well as scratches and other blemishes on the workpiece. Attempts to obviate this problem by increasing the mixing rate of "Part A" and "Part B" have been somewhat unsuccessful, because the cross-linking reaction rate is often faster than even the most sophisticated mixing techniques.
Closed-celled urethane stones are unsatisfactory in several additional regards. For example, the urethane stones comprise an inherently stiff composite structure. Consequently, agglomerates and nodules created from bubble surfaces as they are breached during stone dressing can scratch a workpiece if the nodules are not liberated from the stone and rinsed away. Moreover, both PVA-based stones and urethane-based stones require the use of highly toxic materials in their manufacture. This is particularly true of many isocyanide urethane resins.
Combining phenolic resins with a foaming agent and a catalyst is another well known process for making abrasive foam grinding structures. With this process, the catalyst creates an exothermic reaction which causes the foam to "explode", creating bubbles in the resin which get trapped therein as the resin cures.
Yet another method for making abrasive foam grinding structures is disclosed in U.S. Pat. No. 5,713,968 to Fruitman et al which is incorporated herein by reference. This method involves adding microballoons to a resin mixture during the mixing stages. The friability of the abrasive foam grinding structure can thereby be controlled by adjusting the size and density of the microballoons, as well as by adjusting the amount and composition of diluent, such as deionized water and ethanol, which is added to the resin during the mixing process. However, since the amount of diluent that may be added is limited by the resin to diluent ratio necessary to maintain a satisfactory structure, the potential for controlling the friability of the abrasive foam grinding structure by varying diluent amounts is somewhat limited. Also, this process requires the use of pressurized vessels to prevent air from mixing with the resinous compound and altering the porosity of the structure. For this reason and others, this particular method tends to be expensive to operate.
In summary, most of the presently known abrasive foam grinding structures and techniques for manufacturing them are unsatisfactory in several additional regards. Most known abrasive foam grinding structures do not grind or hone the surface of the workpieces in a uniform fashion, creating unsatisfactory workpiece surface finishes. In addition, imperfect dispersion of abrasives in the stones tends to yield agglomerates or nodules that project from the mean surface of the abrasive foam grinding structures and can cause scratching and surface imperfections in the workpiece. Moreover, the toxic nature of many of the components of known grinding or honing stones renders their manufacture, use, and disposal environmentally compromising and expensive. Finally, the cost of manufacturing most presently known stones is getting increasingly high, exacerbating the problem of limited stone life.
A new abrasive foam grinding structure composition and method for making and using the same is therefore needed which overcomes the shortcomings of the prior art.